Consumers today expect household goods to be safe, secure, healthy and comfortable to use. There is in particular much interest in preventing microbial contamination, which prevention is closely associated with a sense of product safety and security, and a desire for products in one's everyday life to have antibacterial/antifungal qualities.
Antibacterial/antifungal agents can be broadly divided into organic agents and inorganic agents. The synthetic organic antibacterial/antifungal agents that have hitherto been commonly used are inexpensive and effective even in small amounts. Yet, they often exhibit efficacy only against certain microorganisms; the difference in effects on, for example, Gram-negative bacteria, Gram-positive bacteria and molds is sometimes considerable. Additional drawbacks include the ready emergence of resistant organisms, poor heat resistance, and efficacy that is rapid but not long-lasting. There is also a growing concern over the impact of such organic agents on the human body and the environment, which is why inorganic agents are starting to become the norm for antimicrobial agents. However, given the low efficacy of inorganic antifungal agents, most antifungal agents in use today are organic antifungal agents.
Titanium oxide-based photocatalytic materials have recently been attracting attention as inorganic antibacterial/antifungal agents and are increasingly being put to use in substrate surface cleaning, deodorizing, antibacterial and other applications. A photocatalytic reaction is a reaction occurred by excited electrons and holes that generate due to the absorption of light by titanium oxide. Photocatalytic materials are thought to have the following mechanism of action as antibacterial agents: the excited electrons and holes that have formed at the surface of the titanium oxide due to photocatalytic reactions carry out oxidation-reduction reactions with oxygen and water adsorbed to the titanium oxide surface, and the active species thus generated act on microorganisms, causing cell membrane damage and either killing the cells outright or causing them to ultimately break down through long-term action. Advantages of photocatalytic materials thus include their ability to exhibit efficacy on a broad range of microorganisms, including fungi, the low possibility of resistant organisms emerging, and the substantial lack of deterioration in efficacy over time.
Because photocatalytic reactions are triggered by exposure to light in the ultraviolet region (wavelength range: 10 to 400 nm) and light in the visible region (wavelength range: 400 to 800 nm), such efficacy cannot in principle be obtained in dark places untouched by natural light or artificial light. Yet, because bacteria and fungi proliferate even in the absence of light, in products required to have a durable performance over a desired period of time, such as antibacterial/antifungal products, there has existed a desire for photocatalytic materials which exhibit antibacterial/antifungal properties even in dark places that are not exposed to light.
Such challenges are being addressed by investigations on photocatalytic materials that complement the function of a photocatalyst by using the photocatalyst together with an antibacterial/antifungal agent other than a photocatalyst. Photocatalysts break down organic matter, and so the use of an inorganic antibacterial/antifungal agent makes sense. For example, JP-A 2000-051708 and JP-A 2008-260684 disclose that antibacterial properties and antifungal properties even in dark places are achieved by adding silver or copper as an antibacterial/antifungal ingredient.
Photocatalysts are generally used by dispersing photocatalytic particles in a solvent and mixing in a film-forming ingredient so as to form a coating which is then applied onto a substrate. However, as mentioned above, when a metal constituent such as silver, copper or zinc is added to increase the antibacterial/antifungal properties, a number of practical problems arise. Specifically, cases in which the method of supporting a metal such as silver, copper or zinc, or a compound thereof, involves reacting a metal starting material with a photocatalytic particle powder are undesirable because a great deal of effort is required to then disperse the supported catalyst in a solvent. In cases where a metal starting material is added to a dispersion in which photocatalyst particles have already been dispersed, the stability of the dispersed photocatalytic particles is compromised, causing agglomeration to arise. It is thus often difficult in practice to obtain the required transparency when forming such a photocatalytic thin film on various substrates.
In methods for obtaining antibacterial/antifungal metal-containing photocatalytic particles by adding a metal such as silver, copper or zinc, or a compound thereof, to a photocatalyst starting material and then carrying out heat treatment, the crystallinity of the photocatalytic particles decreases, and so the resulting photocatalyst performance declines. Also, because a portion of the antibacterial/antifungal metal is covered by titanium oxide and therefore is not exposed at the surface, the resulting antibacterial/antifungal properties are also diminished.
Another problem is that, when a large amount of a metal such as silver, copper or zinc, or a compound thereof, is added in order to increase the antibacterial/antifungal properties in dark places, a decreased amount of light reaches the photocatalyst, leading to a decline in the resulting catalyst performance. Hence, a photocatalytic thin film having both sufficient antibacterial/antifungal properties for practical use and the transparency required for use on various substrates has yet to exist.
Silver particles have optical, electrical and thermal properties not observed in other substances, and thus are used in various fields. Because they can be expected to exhibit antimicrobial effects even at a relatively low concentration, have a broad antimicrobial spectrum that includes fungi, and also have a high degree of safety in the human body, they are sometimes utilized in antimicrobial coatings. However, silver readily reacts with the sulfur constituent of sulfur compounds such as hydrogen sulfide that are present in trace amounts within the environment to form silver sulfide, leading to associated problems such as discoloration and a weakening of the antimicrobial effects.
According to JP-A 2015-034347, because alloys exhibit qualities that differ from those of the constituent metal elements in unalloyed form, properties unattainable with existing metals alone are expected to emerge with the creation of new alloys. For example, JP-A H11-217638 examines the alloying of silver as a means for increasing the sulfidation resistance of silver.